Scour control system for submerged structures

ABSTRACT

A scour control system for establishing and maintaining convergent fluid flow conditions at the surface of a submerged particulate bed circumjacent to a submerged structure seated on the bed so that particle loss from the surface of the bed, within a closed region surrounding the structure, is eliminated as being the normal consequence of externally incident unidirectional fluid flow past the submerged structure at the particulate bed surface.

United States Patent Bruce 1 Aug. 29, 1972 [54] SCOUR CONTROL SYSTEM FORSUBMERGED STRUCTURES [72] Inventor: Peter Bruce, 1O Torphichen Place,

Edinburgh, EH3 8DU, Scotland I22] Filed: Jan. 13, 1970 [2! 1 Appl. No:2,501

[30] Foreign Application Priority Data Jan. 17, 1969 Great Britain..2,927/69 Jan. 25, 1969 Great Britain ..4,351/69 [52] US. Cl ..61/63,61/2 [51] Int. Cl. ..E02b 3/00 [58] Field of Search ..61/2, 53.74, 1,63; 37/58, 37/75, 78, 79

[56] References Cited UNITED STATES PATENTS 7 2,854,825 10/1958 Crake..61/53.74 X

3,312,069 4/1967 Jorda ..61/1 3,449,915 6/1969 Cummings ..61/2 3,452,9667/1969 Smolski ..61/1

Primary ExaminerDavid J. Williamowsky Assistant Examiner-David H. Corbin[5 7] ABSTRACT A scour control system for establishing and maintainingconvergent fluid flow conditions at the surface of a submergedparticulate bed circurnjacent to a submerged structure seated on the bedso that particle loss from the surface of the bed, within a closedregion surrounding the structure, is eliminated as being the normalconsequence of externally incident unidirectional fluid flow past thesubmerged structure at the particulate bed surface.

17 Claims, 15 Drawing; Figures PATENTEDmszs m2 sum 1 0F 5 Figure I SCOURCONTROL SYSTEM FOR SUBMERGED STRUCTURES Preferably the vertical positionof an upper extremity of an accumulation of particles against thesubmerged structure, and hence the magnitude of the accumula tion ofparticles, is regulated by automatic control means.

Preferably the automatic control means for regulating the magnitude ofthe accumulation of particles includes: at least one sensing elementcapable of providing a signal related to the vertical distanceseparating a reference point on the shroud The present invention relatesto a pump and shroud ducting system for modifying fluid flow patternsaround the intersection of a submerged structure with a submergedparticulate supporting bed in order to mitigate against and control thelocal erosion of the bed adjacent to the structure. Such erosion isgenerally known as scour.

The submerged structure could, for example, be a supporting leg of ajack-up sea drilling platform or rig.

Scour can occur at a submerged particulate bed when a submergedstructure presents an obstruction to fluid flow over the bed and causesan increase in fluid flow velocity at locations on the bed surfaceadjacent to the structure. These locations then experience increasedparticle entrainment with the moving fluid. Particle loss from thelocations ensues with the result that progressive erosion of the surfaceof the bed within these locations occurs.

If the fluid velocity is sufficiently high, surface erosion or scouradjacent to the submerged structure may proceed until the structure isundermined to such an extent that a settlement of the structure into thesurface of the submerged bed may occur.

The supporting legs of sea drilling rigs are particularly prone to suchsettlement due to scour and resultant undesirable stresses in theoverall rig structure are presently commonplace.

Since particle movement at the surface of a submerged particulate bed inthe presence of a fluid flow in the local area adjacent to a submergedstructure will generally follow the fluid flow paths at the surface ofthe bed, it follows that particle loss from such an area will occur ifthe flow paths pass out of the area.

Thus, if the flow paths at the particulate bed surface can beconstrained to converge at all times on the sub merged structure,particle accumulation against the structure will occur and no scouringaction, with resultant undermining of the structure, will be possible.

Such convergent fluid flow paths can be established by providing avertically extending tubular shroud adjacent to the structure andpumping means whereby fluid may be conducted from the particulate bedadjacent to the structure to a level so spaced above the plane of theparticulate bed that discharge of the ducted fluid causes substantiallyno disturbance at the particulate bed.

It is an objective of the present invention to mitigate and control thescouring action that tends to occur at a submerged particulate bedadjacent to a submerged structure by altering the fluid flow paths atthe submerged particulate bed such that an accumulation of particles,transported by fluid flow, tends to occur against the submergedstructure.

1 According to the present invention, a scour control system includes:an elongate, hollow, cylindrical shroud means, of substantially verticalaxial orientation and with an open lower extremity, for conveying fluidfrom a location at the surface of a particulate submerged bed adjacentto a submerged structure seated thereon, to an upper location at a levelso spaced vertically from the particulate bed that discharge of theconveyed fluid at the upper location causes substantially no disturbanceof the particulate bed surface; fluid pumping means connected to theshroud whereby fluid flow may be caused within the shroud means; andmounting member means connecting said shroud means to said submergedstructure whereby the shroud means is vertically substantially coaxialwith the submerged structure and the open lower extremity of the shroudmeans is maintained in a position adjacent to the submerged structureand spaced from the particulate bed surface such that upwards forcedfluid flow within the shroud means, due to the action of the fluidpumping means, causes a convergent fluid flow at the surface of theparticulate bed adjacent to the submerged structure so that anaccumulation of particles, transported by the convergent fluid flow,tends to occur against the submerged structure.

Preferably the shroud means is located coaxial with the vertical orsubstantially vertical axis of the submerged structure.

Preferably the shroud means is located outside, ad jacent to, andsurrounding the submerged structure.

Preferably the shroud means is located inside the submerged structure.

Preferably the shroud means is constructed as an integral part of thesubmerged structure.

Preferably the mounting member means includes a movable mounting whichpermits relative vertical movement of the shroud means with respect tothe submerged structure.

Preferably the vertical position of the shroud means with respect to thesubmerged structure is regulated by automatic control means.

Preferably the mounting member means includes an automatic control meanscomprising at least one sensing element capable of providing a signalrelated to change in the vertical distance separating a reference pointon the shroud means from the plane of the undisturbed submergedparticulate bed; signal amplification means; actuator motor drivingmeans capable of changing the vertical position of the shroud means; andsignal transmission means between the sensing elements, the amplifiermeans, and. the actuator motor driving means whereby negative feedbackcontrol of the position of the shroud means with respect to theundisturbed particulate bed plane may be established.

Preferably the vertical position of an upper extremity of anaccumulation of particles against the submerged structure, and hence themagnitude of the accumulation of particles, is regulated by automaticcontrol means.

Preferably the automatic control means for regulating the magnitude ofthe accumulation of particles includes: at least one sensing elementcapable of providing a signal related to the vertical distanceseparating a reference point on the shroud means from an upper extremityof an accumulation of particles against the submerged structure; signalamplification means; actuator means capable of changing the rate offluid flow through the shroud means; and signal transmission meansbetween the sensing elements, the amplifier means, and the actuatormeans whereby the rate of fluid flow within the shroud means and overthe adjacent particulate bed surface, and hence the rate of accumulationof particles against the submerged structure, may be so regulated bynegative feedback that position control of the vertical location of theupper extremity of the particle accumulation with respect to thereference point on the shroud means may be established.

According to a preferred aspect of the present invention, the fluidpumping means includes a rotatable impeller means mounted coaxiallywithin the shroud means, and turbine means, drivable by externallyincident fluid flow and coupled to the rotatable impeller means suchthat rotation of the turbine means due to the action of externallyincident fluid flow results in an upward fluid movement within theshroud means.

Preferably the turbine means comprises a tangentially drivenaxial-bladed turbine wheel coupled to and mounted coaxially above theimpeller means such that externally incident fluid flow in any directiontangential to the turbine wheel and in the plane of rotation of theturbine wheel causes unidirectional rotation of the turbine wheel.

According to another preferred aspect of the present invention, thefluid pumping means includes at least one nozzle located inside andsubstantially adjacent a lower extremity of the shroud means; at leastone pipe capable of supplying a fluid to the nozzles and connectedthereto; and a supply of a fluid deliverable through the pipe to thenozzles such that release or discharge of the fluid from the nozzlescauses upward fluid movement within the shroud means by entrainment withthe discharge, in the case of a liquid, and by entrainment with risingbuoyant bubbles, in the case of a gaseous or a vaporous fluid deliveredby the pipe. It will be appreciated that in the case of a liquiddischarge from the nozzles, the nozzles must be pointing upwards.

The principle of operation of the scour control system may be describedbest with reference to the diagrammatic representations of FIGS. 1, 2and 3 of the accompanying drawings.

FIG. 1 shows a sectional view through a vertically orientated tubularshroud 1 containing a coaxially mounted impeller 2 driven by a motor 3and vertically separated from the surface of a submerged particulate bed4. Rotation of the impeller 2 by the motor 3 causes upward fluid flowwithin the shroud l which, in turn, causes a particular fluid flowpattern at the surface of the particulate bed 4. The arrowed linesindicate fluid flow paths. A plan representation of the fluid flow paths5 lying in the plane indicated by A-A in FIG. 1 is given in FIG. 2. Forno external disturbances in the fluid overlying the submergedparticulate bed 4, the fluid flow lines 5 in the plan representation ofFIG. 2 will be radially convergent on the axis of the tubular shroud 1.

If a unidirectional fluid flow at the surface of the submergedparticulate bed 4 exists additionally, as an external disturbance, theresultant combination of fluid flow patterns may be depicted by thefluid flow lines 5 shown in the plan representation of FIG. 3.

The region denoted by the broken line 6 may be seen to experience onlyconvergent flow at the particulate bed surface despite the existence ofan external unidirectional fluid flow. Since no net outflow of fluidoccurs across the closed boundary 6 at the surface of the particulatebed 4, no outward transportation of particles with consequent particleloss from the region can occur. Hence, scour, involving particle lossfrom a closed bounded region of a submerged particulate bed due toparticle transportation by overpassing fluid flow, may be mitigated andprevented by the application of a superimposed convergent fluid flowwithin the region prone to scour.

This then is the principle of operation of the present invention.

Embodiments of the present invention will now be described by way ofexample with further reference to the accompanying drawings in which:

FIG. 4 shows a sectional side view of a submerged structure seated on aparticulate bed with a scour control system, according to a firstembodiment of the present invention, located inside the structure;

FIG. 5 shows a sectional plan view through B-B of FIG. 4 with the fluidflow patterns at the surface of the submerged particulate bed shown forthe system in operation;

FIG. 6 shows a sectional side view of a submerged structure with a scourcontrol system, according to a second embodiment of the presentinvention, located external to the structure;

FIG. 7 shows a sectional plan view through CC of the structure and scourcontrol system depicted in FIG.

FIG. 8 shows a sectional side view of a submerged structure seated on aparticulate bed with a scour control system, according to a thirdembodiment of the present invention, located externally on the submergedstructure and including a tangential-drive axial-bladed turbine drivenby external fluid flow and coupled coaxially to an impeller within theshroud part of the scour control system;

FIG. 9 shows a sectional plan view through D-D of the submergedstructure and scour control system depicted in FIG. 8;

FIG. 10 shows a perspective view of the third embodiment of the presentinvention shown in FIG. 8;

FIG. 11 shows a sectional side view of a submerged structure with ascour control system, according to a fourth embodiment of the presentinvention, located externally on the structure and including fluiddischarge nozzles mounted inside the shroud part of the scour controlsystem;

FIG. 12 shows a sectional side view of a submerged structure with ascour control system, according to a modification of the fourthembodiment of the present invention, constructed integral with thesubmerged structure and with powering fluid discharge nozzles mountedwithin the submerged structure wall which acts additionally as theshroud part of the scour control system;

FIG. 13 shows a diagrammatic representation of a scour control system,according to the fourth embodiment of the present invention as shown inFIG. 11, including a position control system for regulating the verticalposition of the shroud part of the scour control system;

FIG. 14 shows a diagrammatic representation of a scour control system,according to the fourth embodiment of the present invention as shown inFIG. 11, including a position control system for regulating theproximity of the upper extremity of the particle accumulation againstthe submerged structure to the lower extremity of the shroud part of thescour control system;

FIG. shows a diagrammatic representation of an electrical circuitconfiguration included in the position control system depicted in FIG.14.

Referring to FIGS. 4 and 5, in a first embodiment of the presentinvention, a scour control system is located within a submergedopen-frame structure 1a and includes an elongate hollow circularcylindrical openended shroud 1, substantially vertically orientatedaxially and with an internal centrally mounted bladed impeller 2 drivenby an electric motor 3, movably mounted by brackets7 and pinions 8engaging with rails 9 fixed to the submergedstructure 1a which is seatedon a submerged particulate bed 4 and penetrates below the undisturbedsurface plane 4a of the bed 4. An adjustable cable 10, attached by oneend to an uppermost one of the brackets 7 and by the other end to ananchorage point on an upper part of the structure la, serves to locatethe shroud 1 vertically with respect to the structure la in a positionabove the plane 4a such that upwards movement of fluid within the shroud1 due to rotation of the impeller 2 by the electric motor 3 establishesflow paths 5, despite the presence of external unidirectional fluid flowpaths 5a, whereby a particle accumulation 4b tends to occur against thestruc ture la.

Referring to FIGS. 6 and 7, in a second embodiment of the presentinvention, a scour control system is located externally around asubmerged circular cylindrical structure lg and includes an elongatehollow circular cylindrical annular shroud lb, open at a lower extremitylie and closed at an upper extremity 1d. A horizontal tubular duct 1ecommunicates the interior space of the annular shroud lb with theexternal fluid. A bladed impeller 2 driven by an electric motor 3 ismounted within the duct 1e at the open end remote from the shroud lb. Aplanar fin 1f, vertically orientated, forms a web between the duct 1eand the shroud lb to act as a direction orienting vane. The annularshroud lb is movably mounted on the structure lg by captive bearingballs 11 which are held captive with respect to the shroud lb and spaceapart the shroud lb and the structure lg. An adjustable cable 10,attached by one end to the upper external surface of duct 1e and by theother end to an anchorage point on an upper part of the structure lg,serves to locate the shroud lb vertically with respect to the structurelg such that upwards movement of fluid within the shroud 1g due to therotation of the impeller 2 by the electric motor 3 establishes flowpaths 5, despite the presence of external unidirectional fluid flowsalong flow paths 5a, whereby a particle accumulation 4b may occuragainst the structure 1g.

Referring to FIGS. 8, 9, and 10, in a third embodiment of the presentinvention, a scour control system is located externally around asubmerged circular cylindrical vertical structure lg and includes anelongate, hollow, annular, circular cylindrical shroud 1b, open at alower extremity 1c and with an inner wall 14 slidably bearing againstthe exterior of the structure 1g. A conical hollow annular duct 12closes the upper extremity 1d of the shroud 1b and communicates theannular interior space of the shroud lb with a hollow cylindricalannular duct 13 containing a bladed impeller 2 rotatably mounted onbearings 16 around the inner wall 14 of the shroud lb. A tangentiallydriven axialbladed circular cylindrical turbine wheel 15, carrying axialblades 17, is mounted coaxially above the impeller 2 and is directlycoupled to it while additionally being rotatably mounted coaxiailyaround the inner wall 14 of the annular shroud lb such that externalfluid flow incident on the blades 17 of the turbine wheel 15 causesunidirectional rotation 18 of the turbine wheel 15 and consequentunidirectional rotation 18 of the impeller 1 which, in turn, causesupward fluid movement within the shroud lb, the duct 12, the duct 13,and the turbine wheel 15 whereat centrifugal force causes ejection ofthe ducted fluid out between the rotating turbine blades 17 as aneffluence 19. This upward fluid movement causes ingestion of fluid atthe open ,lower extremity 1c of the shroud lb such that a convergentfluid flow pattern combines with any external unidirectional flowpattern at the surface of the particulate bed to form a combination flowpattern,as indicated in FIG. 10, which mitigates against and preventstheoccurance of particle removal, or scour, in the region of theparticulate bed 4 circumjacent to the structure lg. As shown in FIG. 8,the horizontal crosssectional area of ducted fluid at the impeller 2 isgreater than the horizontal cross-sectional area of ducted fluid at thelower extremity 1c of the shroud lb so that the velocity of fluidingestion exceeds the velocity of ducted fluid at the impeller 2. Thiscon stitutes a velocity ratio which allows ingestion fluid velocities atthe particulate bed surface circumjacent the structure lg to exceed thevelocity of the incident turbine-driving external fluid flow so as tomaximize the particle transportation into the bed region circumjacentthe structure lg for a given external fluid flow rate and a given totalturbine blade area.

Referring to FIG. 11, in a fourth embodiment of the present invention, ascour control system includes an elongate, hollow, circular cylindrical,annular shroud 1b, open at both upper and lower extremities and slidablylocated externally surrounding a submerged circular cylindricalstructure 1g. A hollow toroidal ring 20 is located and mounted in theinternal annular space within the lower extremity 1c of the annularshroud 1b and encircling the inner wall 14 of the shroud lb. The hollowring 20 is equipped with upwards pointing nozzles 21 equally spacedaround the ring such that pressurized air delivered into the ring 20 maybe discharged upwards in an even distribution within the annular spaceinside the shroud 1b. A pipe 22 supplies pressurized air from an upperlocation on the submerged structure lg to the interior of the ring 20.An adjustable cable 10, attached by one end to the inner wall 14 of theshroud 1b and by the other end to an anchorage point located on an upperpart of the structure lg, serves to locate the shroud 1b vertically withrespect to the structure 1g such that upwards movement of fluid withinthe shroud lb, due to entrainment with rising air bubbles 25 issuingfrom the nozzles 21, establishes flow paths 5, despite the presence ofexternal unidirectional fiows, whereby a particle accumulation 4b maytend to occur against the structure lg.

Referring to FIG. 12, in a modification of the fourth embodiment of thepresent invention as shown in FIG. 11, a scour control system includes ashroud constructed as an integral part of a submerged cylindricalstructure lg. Horizontal rows of circumferencially disposed equallyspaced slanted inlet holes 23 are located in the wall of the structurelg at several equally spaced vertical locations. A movable cylindricalsleeve 24 capable of closing off all of the holes 23 is suspended withinthe structure lg by an adjustable cable 10 of which one end is attachedto the sleeve 24 and the other end is attached to an anchorage pointlocated on an upper part of the structure lg.

The purpose of the sleeve 24 is to uncover successively higher inletholes 23 as the structure 1g settles into the submerged bed 4, under theforce of its own weight, so that the scour control system caneffectively maintain a working separation from the particulate bedsurface plane 4a.

A hollow toroidal ring is fixed to the inside of the wall of thestructure lg above the uppermost reach of the sleeve 24 and is equippedwith equally spaced upwards pointing nozzles 21 whereby pressurized airsupplied by a pipe 22 attached to the ring 20 may be discharged withinthe structure lg so that rising buoyant bubbles 25 are formed whichentrain fluid while rising up within the structure lg to escape fromexhaust holes 26 located above the ring 20. The resultant upwards fluidmovement causes fluid ingestion at the uncovered inlet holes 23 wherebya scour mitigation and prevention effect is established at the surfaceof the particulate bed circumjacent to the submerged structure lg ashereinbefore described.

Referring to FIG. 13, in an embodiment of a first aspect of the presentinvention, a scour control system as shown in FIG. 11 includes a meansof automatically positioning the shroud lb with respect to theundisturbed particulate bed plane 4a which comprises: a depth measuringdevice 27, of the acoustic echo sounder type, capable of giving anoutput voltage proportional to the vertical distance between the device27 and the particulate bed plane 4a; a differential amplifi er 29 tocompare the output voltage of device 27 with a remotely selectablereference voltage and amplify the difference between these two voltages;an actuator 30 driven by the output of amplifier 29; an electric motor31 controlled by the actuator 30; a winch drum 32 mounted on the spindleof the electric motor 31; and a cable 10 anchored to and wrapped roundthe drum 32 at its upper end and attached to an upper part of themovable shroud lb of the scour control system by its lower end. Thedepth measuring device 27 is mounted, together with the amplifier 29, onone extremity of a horizontal elongate member 28 attached rigidly to theshroud 1b so that the device 27 operates over an undisturbed area of thesurface of the particulate bed 4. The actuator 30, motor 31, and drum 32are mounted on an upper part of the submerged structure lg. The shroudlb is drawn upwards by the cable 10 winding on the drum 32 when thevoltage from device 27 is less than the reference voltage and is lowereddownwards when the voltage from device 27 exceeds the reference voltage.Thus variation of the reference voltage may be used to set the steadystate vertical position of the shroud, 1b with respect to the plane 4a.Flexible interconnecting wires 32a electrically link the amplifier 29,the depth measuring device 27, and the actuator 30 such that, togetherwith the motor 31, the drum 32, the cable 10, and the movable shroud lb,a negative feedback closed loop automatic position control system isformed capable of maintaining the shroud lb at a selectable level abovethe undisturbed particulate bed plane 4a.

Referring to FIGS. 14 and 15, in an embodiment of a second aspect of thepresent invention, a scour control system as shown in FIGS. 11 and 13includes a means of automatically regulating the vertical position ofthe upper extremity 4d of the accumulation of particles 4b, occurringagainst the submerged structure lg supporting the shroud lb of the scourcontrol system, which includes: a photo-electric level sensing device33, which produces a voltage proportional to the vertical separationbetween the lower extremity of the shroud 1b and the upper extremity ofthe particle accumulation 4b, the lastmentioned upper extremity beingdesignated 4d; a differential amplifier 34, mounted on the shroud lb,which compares the output voltage of device 33 with a selectablereference voltage and amplifies the difference between the two voltages;an electric lamp 35 mounted on the lower extremity 1c of the shroud lband orientated to illuminate the entire sensing device 33 in the absenceof obscuration of device 33 by particle accumulation; an electromagneticactuator valve 36 capable of varying the supply of air through pipe 22to the ring 20 and nozzles 21 of a scour control system according to thefourth embodiment of the present invention hereinbefore described; andinterconnecting wires 37 linking the sensing device 33, the amplifier34, and the actuator valve 36 together to form a closed loop negativefeedback automatic position control system capable of maintaining thelevel of the upper extremity 4d of the particle accumulation 4b at aselectable distance below the lower extremity of the shroud lb.

The photo-electric level sensing device 33 comprises an elongate rigidmember 38 attached to and depending vertically from the inner wall 14 ofthe shroud lb. An array of photodiodes 39, equally spaced verticallyalong the member 38, are connected together by the cathodes to anegative voltage supply 40. The anode of each photodiode 39 is connectedbetween successive resistors 41 in a chain of equal resistors which inturn is connected between the negative voltage supply 40 and thepositive voltage supply 42. Illumination of any of the photodiodes 39causes it to drop to a low value of reverse impedance and hence toshort-circuit the corresponding resistor in the resistor chain. Thephotodiodes 39 adjacent to the negative voltage supply 40 are located atthe uppermost part of the support member 38 so that illumination by lamp35 of those photodiodes 39 unobscured by the particle accumulation 4bcauses short-circuiting of the corresponding resistors 41 in theresistor chain such that an output voltage from a point 43 on theresistor chain exhibits a stepped variation in proportion to heseparation of the level of obscuration of the photodiodes 39 from thelower extremity 1c of the shroud 1 lb.

The output of the level sensing device 33 is connected to one input of adifferential amplifier 44, incorporating a stabilization network, whereit is compared with a reference voltage selectable by means of apotentiometer 45 connected across the positive and negative voltagesupplies 42 and 40. The output of the amplifier M is connected to drivethe actuator valve 36 which in turn varies the rate of supply of airpassing through the pipe 22 and, hence, the rate of fluid flow entrainedwithin the shroud 1b. The setting of the potentiometer 45 will determinethe steady state position of the level of obscuration of the photodiodes39 and hence will determine the steady state level of the upperextremity 4d of the particle accumulation 4b.

With the scour control system in open loop opera tion, that is, withoutthe negative feedback provided by the automatic control systems of thefirst and second aspects of the present invention, the accumulation ofparticles 4b may increase until the lower part of the shroud l or lbtends to become blocked. In this condition, the necessary convergentfluid flow at the surface of the particulate bed 4 will decrease tonegligibly small proportions permitting the existing natural externalfluid flow, tending to cause scour, to erode the particle accumulation4b until the lower part of the shroud 1 or lb becomes cleared ofparticulate obstruction. An equilibrium balance state or an oscillatorystate will tend to be established between these two opposing influenceswith the result that the upper extremity 4d of the particle accumulation4b will be maintained within limits about a level below the lowerextremity 1c of the shroud l or lb related to fluid flow rate within theshroud and to external fluid flow rate over the surface of theparticulate bed 4 circumjacent to the submerged structure la or lg.

Both control systems hereinbefore described may advantageously beincluded in a single scour control system. I

Such open loop operation of the scour control system can, however,neither accommodate for settlement of the submerged structure due tocauses unrelated to scour nor eliminate the possibility of reaching anunstable oscillatory state of the particle accumulation.

With the scour control system in closed loop operation, that is,including the negative feedback provided by the automatic controlsystems, the upper extremity 4d of the particle accumulation 4b will bemaintained in a substantially fixed positional relationship to theshroud l and lb which, in turn, will be in a substantially fixed stablepositional relationship with the undisturbed particulate bed plane 4a.

Any desired accumulation of particles 4b against the submerged structurela and lg may therefore be maintained in a stable manner irrespective ofsettlement of the supporting submerged structure la or 13 or the rate offlow of the external fluid past the structure la or lg at theparticulate bed surface 40.

It will be appreciated that the pumping means of the scour controlsystem may take a variety of additional forms, all with the object ofcausing upward fluid flow within the shroud of the scour control system.

Also, it will be appreciated that alternative locations of the shroudare possible. For example, the shroud may be located upstream andadjacent to the submerged structure; the shroud may alternatively bereplaced by a plurality of shrouds, serving a single impeller or eachwith an impeller, disposed adjacent to the submerged structure and abovethe submerged particulate bed. In each of these last mentioned examples,the action of the scour control system is substantially as hereinbeforedescribed.

It may be noted that reversal of the impeller motion or the impellerblade angle of attack will reverse the fluid flow in the shroud and socause DlVERGENT fluid flow paths at the submerged particulate bedsurface. The reversed fluid flow paths will now leave any local closedarea of the particulate bed circumjacent to the submerged structure, atthe surface of the bed, with the result that massive particle loss fromthe local area of the bed will occur constituting accelerated scouring.This mode of operation of the scour control system according to thepresent invention may be used to free a submerged structure penetratinga submerged particulate bed from the gripping action of the bed and sofacilitate easy removal of the structure from its supporting bed.

In the operation of the scour control system according to the presentinvention, say for controlling scour at a leg of a jack-up bottomsupported sea drilling rig, the shroud 1b is approximately located withrespect to the sea bed surface 4a when the leg lg is positioned on thesea bed 4. The shroud llb is then lowered by the cable 10 until it isalmost in contact with the surface 40 of the sea bed 4. The air supplyto the nozzles 21 on ring 20 within shroud 1b is set at a low deliveryrate until an accumulation of sand particles 4b builds up sufficiently,due to the water flow 5, to partially block the lower extremity 1c ofthe shroud 1b. The shroud lb is now raised slowly and the delivery rateof air to the nozzles 21 increased until the sand particle accumulationagain partially blocks the lower extremity lc of the shroud lb with theair delivery rate finally set at half the max imum rate. This then isthe normal operating state of the scour control system and theconvergent water flow condition at the surface of the sea bed, necessaryto mitigate against and prevent scouring, is established.

Where the scour control system includes the two automatic controlsystems for regulating the position of the shroud lb and the proximityof the upper extremity 4d of the sand accumulation 4b to the lowerextremity 1c of the shroud lb, the operation of the scour control systemis considerably simplified and consists merely of energizing bothcontrol systems and preselecting the two reference voltages to thedifferential amplifiers 29, 34. The shroud 1b will then be drivendownwards by motor .31 automatically until device 27 produces a signalvoltage output equal to the reference voltage selected for amplifier 29whereupon the shroud lb will stop and be maintained in its verticalposition, owing to the disappearance of the amplified difference voltageat-the output of the differential amplifier 29 causing the motor 31 tostop. The shroud will thus be maintained at a constant height above thesea bed surface according to the magnitude of the preselected referencevoltage. The sensing device 33, of the second automatic control system,may have penetrated the sea bed surface or may still be wholly above thesea bed surface. in the latter case, the water flow rate within theshroud is at a maximum and in the former case at a lesser rate accordingto the degree of obscuration of the sensing device 33 by the sea bedmaterial. The water flow into the shroud lb causes a build-up of sandagainst the submerged structure lg until increasing sand obscuration ofthe sensing device 33 changes its output voltage such that the referencevoltage to the amplifier 34 is equalled and the water flow consequentlycut off due to closure of the actuator valve 36 for zero signal out ofthe amplifier 34. Any external scouring action opposes the increase ofthe sand accumulation 4b, and hence the rise of the obscuration level ofdevice 33, so that the dynamic equilibrium state involves the existenceof a voltage error between the output of the device 33 and the referencevoltage to amplifier 34. This error voltage, when amplified by amplifier34, constitutes the demand signal to the actuator valve 36 of theautomatic control system which maintains an appropriate water flow ratewithin the shroud lb such that the necessary convergent flow conditionsfor scour mitigation and prevention are always present during operationof the scour control system.

Thus, the operation of embodiments of the present invention makespossible the elimination of scouring as a hazard to bottom supportedsubmerged structures at sea.

I claim:

1. A scour control system including: an elongate, hollow, cylindricalshroud means, of substantially vertical axial orientation and with anopen lower extremity, for conveying fluid from a location at the surfaceof a particulate submerged bed, adjacent to a submerged structure seatedon the submerged bed, to an upper location at a level so spacedvertically from the surface of the particulate bed that discharge offluid from the shroud at the upper location causes substantially nodisturbance of the particulate bed surface; fluid pumping meansconnected to the shroud means whereby fluid flow may be caused withinthe shroud means; mounting member means connecting said shroud meansmovably to said submerged structure whereby the shroud means isvertically substantially coaxial with the submerged structure and theopen lower extremity of the shroud means is maintained in a positionadjacent to the submerged structure and spaced from the particulate bedsurface such that upward forced fluid flow within the shroud means, dueto the action of the fluid pumping means, causes a convergent fluid flowat the surface of the particulate bed adjacent to the submergedstructure so that an accumulation of particles, transported by theconvergent fluid flow, tends to occur against the submerged structure;and automatic control means whereby the vertical position of an uppersurface of any accumulation of particles against the submerged structurearising from fluid ingestion into the shroud means, and hence themagnitude of such accumulation of particles, is substantially regulatedand controlled.

2. A scour control system as claimed in claim 1, wherein the fluidpumping means includes: at least one nozzle located inside andsubstantially adjacent a lower part of the shroud means; at least onepipe connected to the nozzle and capable of supplying a fluid to thenozzle; mounting means whereby the nozzle is mounted within the shroudmeans; and a supply of fluid deliverable through the pipe to the nozzlesuch that release or discharge of the fluid from the nozzle causesupward fluid movement within the shroud means by entrainment with thedischarge, in the case of a liquid supply fluid, and by entrainment withrising buoyant bubbles, in the case of a gaseous or vaporous supplyfluid.

3. A scour control system as claimed in claim 2, wherein the automaticcontrol means includes: at least one sensing element capable ofproviding a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the particulatebed adjacent surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical location of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.

4. A scour control system as claimed in claim 2, wherein the verticalposition of the shroud means with respect to the submerged structure isregulated and controlled by automatic control means.

5. A scour control system as claimed in claim 4, wherein the automaticposition control means of the shroud means includes: at least onesensing element capable of providing a signal related to change in thevertical distance separating a reference point on the shroud means fromthe plane of the undisturbed submerged particulate bed; signalamplification means; actuator motor driving means capable of changingthe vertical position of the shroud means; and signal transmission meansbetween the sensing element, the amplifier means, and the actuator motordriving means whereby negative feedback control of the position of theshroud with respect to the undisturbed particulate bed surface plane maybe established.

6. A scour control system as claimed in claim 1, wherein the fluidpumping means includes a rotatable impeller means mounted coaxiallywithin the shroud means, and a turbine means drivable by externallyincident fluid flow and coupled to the rotatable impeller means suchthat rotation of the turbine due to the action of the external fluidflow results in an upward fluid movement within the shroud means.

7. A scour control system as claimed in claim 6, wherein the turbinemeans comprises a tangentially driven axial-bladed turbine wheel coupledto and mounted coaxially above the impeller means whereby externallyincident fluid flow in a direction tangential to the turbine wheelcauses unidirectional rotation of the turbine wheel and consequentunidirectional rotation of the coupled impeller means.

8. A scour control system as claimed in claim 7, wherein the automaticcontrol means includes: at least one sensing element capable ofproviding a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the adjacentparticulate bed surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical location of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.

9. A scour control system as claimed in claim 8, wherein the verticalposition of the shroud means with respect to the submerged structure isregulated and controlled by automatic control means.

10. A scour control system as claimed in claim 9, wherein the automaticposition control means of the shroud means includes: at least onesensing element capable of providing a signal related to change in thevertical distance separating a reference point on the shroud means fromthe plane of the undisturbed submerged particulate bed; signalamplification means; actuator motor driving means capable of changingthe vertical position of the shroud means; and signal transmission meansbetween the sensing element, the amplifier means, and the actuator motordriving means whereby negative feedback control of the position of theshroud with respect to the undisturbed particulate bed surface plane maybe established.

11. A scour control system as claimed in claim 1, wherein the automaticcontrol means includes: at least one sensing element capable ofproviding a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the particulatebed adjacent surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical location of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.

12. A scour control system as claimed in claim 11, wherein the verticalposition of the shroud means with respect to the submerged structure isregulated and controlled by automatic control means.

13. A scour control system as claimed in claim 12, wherein the automaticposition control means of the shroud means includes: at least onesensing element capable of providing a signal related to change in thevertical distance separating a reference point on the shroud means fromthe plane of the undisturbed submerged particulate bed; signalamplification means; ac-

tuator mot r drivin means ca able of chan in the vertical position oftfie shroud means; and sign%l t ransmission means between the sensingelement, the amplifier means and the actuator motor driving meanswhereby negative feedback control of the position of the shroud withrespect to the undisturbed particulate bed surface plane may beestablished.

14. A scour control system as claimed in claim 1, wherein the fluidpumping means includes: rotatable impeller means located within an upperextension of the shroud means and coupled to motor means wherebyrotation of the impeller means by the motor means causes an upward fluidmovement within the shroud means.

15. A scour control system as claimed in claim 14, wherein the automaticcontrol means includes: at least one sensing element capable ofproviding a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the particulatebed adjacent surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical lo cation of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.

16. A scour control system as claimed in claim 15, wherein the verticalposition of the shroud means with respect to the submerged structure isregulated and controlled by automatic control means.

17. A scour control system as claimed in claim 16, wherein the automaticposition control means of the shroud means includes: at least onesensing element capable of providing a signal related to change in thevertical distance separating a reference point on the shroud means fromthe plane of the undisturbed submerged particulate bed; signalamplification means; actuator motor driving means capable of changingthe vertical position of the shroud means; and signal transmission meansbetween the sensing element, the amplifier means, and the actuator motordriving means whereby negative feedback control of the position of theshroud with respect to the undisturbed particulate bed surface plane maybe established.

1. A scour control system including: an elongate, hollow, cylindricalshroud means, of substantially vertical axial orientation and with anopen lower extremity, for conveying fluid from a location at the surfaceof a particulate submerged bed, adjacent to a submerged structure seatedon the submerged bed, to an upper location at a level so spacedvertically from the surface of the particulate bed that discharge offluid from the shroud at the upper location causes substantially nodisturbance of the particulate bed surface; fluid pumping meansconnected to the shroud means whereby fluid flow may be caused withinthe shroud means; mounting member means connecting said shroud meansmovably to said submerged structure whereby the shroud means isvertically substantially coaxial with the submerged structure and theopen lower extremity of the shroud means is maintained in a positionadjacent to the submerged structure and spaced from the particulate bedsurface such that upward forced fluid flow within the shroud means, dueto the action of the fluid pumping means, causes a convergent fluid flowat the surface of the particulate bed adjacent to the submergedstructure so that an accumulation of particles, transported by theconvergent fluid flow, tends to occur against the submerged structure;and automatic control means whereby the vertical position of an uppersurface of any accumulation of particles against the submerged structurearising from fluid ingestion into the shroud means, and hence themagnitude of such accumulation of particles, is substantially regulatedand controlled.
 2. A scour control system as claimed in claim 1, whereinthe fluid pumping means includes: at least one nozzle located inside andsubstantially adjacent a lower part of the shroud means; at least onepipe connected to the nozzle and capable of supplying a fluid to thenozzle; mounting means whereby the nozzle is mounted within the shroudmeans; and a supply of fluid deliverable through the pipe to the nozzlesuch that release or discharge of the fluid from the nozzle causesupward fluid movement within the shroud means by entrainment with thedischarge, in the case of a liquid supply fluid, and by entrainment withrising buoyant bubbles, in the case of a gaseous or vaporous supplyfluid.
 3. A scour control system as claimed in claim 2, wherein theautomatic control means includes: at least one sensing element capableof providing a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the particulatebed adjacent surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical location of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.
 4. A scour control system asclaimed in claim 2, wherein the vertical position of the shroud meanswith respect to the submerged structure is regulated and controlled byautomatic control means.
 5. A scour control system as claimed in claIm4, wherein the automatic position control means of the shroud meansincludes: at least one sensing element capable of providing a signalrelated to change in the vertical distance separating a reference pointon the shroud means from the plane of the undisturbed submergedparticulate bed; signal amplification means; actuator motor drivingmeans capable of changing the vertical position of the shroud means; andsignal transmission means between the sensing element, the amplifiermeans, and the actuator motor driving means whereby negative feedbackcontrol of the position of the shroud with respect to the undisturbedparticulate bed surface plane may be established.
 6. A scour controlsystem as claimed in claim 1, wherein the fluid pumping means includes arotatable impeller means mounted coaxially within the shroud means, anda turbine means drivable by externally incident fluid flow and coupledto the rotatable impeller means such that rotation of the turbine due tothe action of the external fluid flow results in an upward fluidmovement within the shroud means.
 7. A scour control system as claimedin claim 6, wherein the turbine means comprises a tangentially-drivenaxial-bladed turbine wheel coupled to and mounted coaxially above theimpeller means whereby externally incident fluid flow in a directiontangential to the turbine wheel causes unidirectional rotation of theturbine wheel and consequent unidirectional rotation of the coupledimpeller means.
 8. A scour control system as claimed in claim 7, whereinthe automatic control means includes: at least one sensing elementcapable of providing a signal related to the vertical distanceseparating a reference point on the shroud means from an upper extremityof any accumulation of particles, within range of the sensing element,which may form against the submerged structure arising from fluidingestion into the shroud means; signal amplification means; actuatormeans capable of regulating the pumping means to change the rate offluid flow through the shroud means; and signal transmission meansbetween the sensing elements, the amplifier means, and the actuatormeans whereby the rate of fluid flow within the shroud means and overthe adjacent particulate bed surface, and hence the rate of accumulationof particles against the submerged structure, may be so regulated bynegative signal feedback that position control of the vertical locationof an upper extremity of the particle accumulation may be establishedwith respect to the reference point on the shroud means.
 9. A scourcontrol system as claimed in claim 8, wherein the vertical position ofthe shroud means with respect to the submerged structure is regulatedand controlled by automatic control means.
 10. A scour control system asclaimed in claim 9, wherein the automatic position control means of theshroud means includes: at least one sensing element capable of providinga signal related to change in the vertical distance separating areference point on the shroud means from the plane of the undisturbedsubmerged particulate bed; signal amplification means; actuator motordriving means capable of changing the vertical position of the shroudmeans; and signal transmission means between the sensing element, theamplifier means, and the actuator motor driving means whereby negativefeedback control of the position of the shroud with respect to theundisturbed particulate bed surface plane may be established.
 11. Ascour control system as claimed in claim 1, wherein the automaticcontrol means includes: at least one sensing element capable ofproviding a signal related to the vertical distance separating areference point on the shroud means from an upper extremity of anyaccumulation of particles, within range of the sensing element, whichmay form against the submerged structure arising from fluid ingestioninto the shroud means; signal amplification means; actuator meanscapable of regulating the pumping means to change the rate of fluid flowthrough the Shroud means; and signal transmission means between thesensing elements, the amplifier means, and the actuator means wherebythe rate of fluid flow within the shroud means and over the particulatebed adjacent surface, and hence the rate of accumulation of particlesagainst the submerged structure, may be so regulated by negative signalfeedback that position control of the vertical location of an upperextremity of the particle accumulation may be established with respectto the reference point on the shroud means.
 12. A scour control systemas claimed in claim 11, wherein the vertical position of the shroudmeans with respect to the submerged structure is regulated andcontrolled by automatic control means.
 13. A scour control system asclaimed in claim 12, wherein the automatic position control means of theshroud means includes: at least one sensing element capable of providinga signal related to change in the vertical distance separating areference point on the shroud means from the plane of the undisturbedsubmerged particulate bed; signal amplification means; actuator motordriving means capable of changing the vertical position of the shroudmeans; and signal transmission means between the sensing element, theamplifier means and the actuator motor driving means whereby negativefeedback control of the position of the shroud with respect to theundisturbed particulate bed surface plane may be established.
 14. Ascour control system as claimed in claim 1, wherein the fluid pumpingmeans includes: rotatable impeller means located within an upperextension of the shroud means and coupled to motor means wherebyrotation of the impeller means by the motor means causes an upward fluidmovement within the shroud means.
 15. A scour control system as claimedin claim 14, wherein the automatic control means includes: at least onesensing element capable of providing a signal related to the verticaldistance separating a reference point on the shroud means from an upperextremity of any accumulation of particles, within range of the sensingelement, which may form against the submerged structure arising fromfluid ingestion into the shroud means; signal amplification means;actuator means capable of regulating the pumping means to change therate of fluid flow through the shroud means; and signal transmissionmeans between the sensing elements, the amplifier means, and theactuator means whereby the rate of fluid flow within the shroud meansand over the particulate bed adjacent surface, and hence the rate ofaccumulation of particles against the submerged structure, may be soregulated by negative signal feedback that position control of thevertical location of an upper extremity of the particle accumulation maybe established with respect to the reference point on the shroud means.16. A scour control system as claimed in claim 15, wherein the verticalposition of the shroud means with respect to the submerged structure isregulated and controlled by automatic control means.
 17. A scour controlsystem as claimed in claim 16, wherein the automatic position controlmeans of the shroud means includes: at least one sensing element capableof providing a signal related to change in the vertical distanceseparating a reference point on the shroud means from the plane of theundisturbed submerged particulate bed; signal amplification means;actuator motor driving means capable of changing the vertical positionof the shroud means; and signal transmission means between the sensingelement, the amplifier means, and the actuator motor driving meanswhereby negative feedback control of the position of the shroud withrespect to the undisturbed particulate bed surface plane may beestablished.